The most common refraction target used by optometrists or ophthalmologists today is the Snellen letter eye chart. It consists of a list of letters typically arranged in a row, or a column. The letter size is also linked to the visual acuity level. For example, letters of font size having the height measurement of the letter N or H of 8.9 mm, it is commonly used to measure a level of 20/20 visual acuity, when such letters are placed at 20 feet away from the test subject. For the 20/10 acuity letters, the letter size is scaled by a factor of two smaller. On the other hand, the 20/40 letters would have twice the size of those of 20/20, etc.
Such eye chart is popular among eye care professional due to its low cost, easy to make, and it works for most purposes. However, a drawback of such type of eye chart is that patients can memorize the letters. Also, each letter has a different effectiveness, that is, some letters are easier than others to read. Moreover, the letters have their own intrinsic orientation. Each letter includes a certain unique configuration of “strokes” presenting a directional preference. For example, the letter H favors the vertical direction, and E has three horizontal lines and one vertical line, etc. Another drawback is that the letters are in the English language, such that the test results may differ depending on the fluency of the test subject with the English language.
Other types of eye charts include tumbling E and Landolt C. Both offer certain benefits, however, neither are used as a refraction target, because they contain only E's and C's, respectively. Therefore, both require an additional directional response to indicate if a test subject can identify the orientation of a symbol correctly. For a quick check of astigmatism, there is yet another type of chart typically used to indicate the existence of cylinder refractive error. It consists of line pairs, or thick lines, typically 12 pairs arranged in 30 degree increments, like the spokes of a wheel. A typical line width in an astigmatism chart is about 2 mm or greater. A thick line or line pairs meet at or near the center of the spoke pattern. In some cases, the spokes stop when they touch or merge with the next spoke or otherwise leave a small central blank zone. Since astigmatism refractive errors are asymmetric in nature, it causes a non-uniform appearance of the spokes, that is, some spokes appears darker than others. A conventional astigmatism chart does not provide visual acuity level information. It is hardly used as a refraction target, nor does it have the sensitivity of a Snellen eye chart when used as a refraction target.
It is desired to have a refraction target, or an eye chart, that is universally usable for all ages from child to adult, without language barriers, for literate or illiterate, convenient, low cost, and easily available to eye care professionals, as well as having no directional preference (unless it is specifically desired to test for cylinder error). Since a desired viewing target is to be used not only for acuity measurement, but also as a refraction target, to arrive at a more accurate refraction end point, it is also desired that such target exhibit higher discrimination sensitivity compared to a Snellen or letter eye chart.